Composites of metal and resin are used in a variety of industrial fields. For example, a metal layer and a resin layer can be joined together by an adhesive material. Recently, a new method of injection joining for manufacturing composites of metal and resin has be introduced. In this method, a molten resin material is injected onto a metal piece that has been previously inserted into an injection molding mold. The surface of the metal combines with the resin. Before inserting the metal piece into the mold, a surface of the metal piece is treated by an etchant to form a number of micropores so that a stronger bond may be formed with the resin material.